Root-mean square volt meter for low frequency currents



April 25, 1961 H. F. HICKS, JR 2,981,887

ROOT-MEAN SQUARE vom METER EoR Low FREQUENCY cuRRENTs Filed June 4, 1954 ATTORNEYS United ROOT-MEAN SQUARE VOLT METER FOR LOW FREQUENCY CURRENTS Harry Frank Hicks, Jr., Rochester, N.Y., assignor, by mesne assignments, tothe United States of America as represented by the Secretary of the Navy Filed June4, 1954, ser. No. 434,683

.6 claims. (ci. 324-98) In investigating the various sources of stray electrical interference andthe effect of such interference upon the performance of electronic equipment, Vespecially under low-signal input conditions, it is frequently desirable to measure the value of this stray interference, or noise, a-s it is commonly termed, at the input of the particular equipment. While devices for measuring the peak amplitudes of such noise areavailable, the information afforded v has an extremely irregular and unpredictable wave form..

`The problem of determining the R.M.S. value of this `noise has, therefore, posed a rather serious problem which has not heretofore been satisfactorily solved. ThisV is particularlyA true as regards the energy present in the very low frequency components of the total noise energy.

It is, therefore, an object of this inventon to provide an apparatus for determining the effective or R.M.S. amplitude of the very low frequency components present in such stray electrical noise.

Patent O "P A further object is to provide such a device which utilizes the variation in resistance caused by the heating effect of a low frequency input signal of unknown or random wave form upon a nonlinear, thermally sensitive resistance of the type commonly` known as a thermistor.

A still further object is to provide such an apparatus in which a thermistor of relatively long thermal time con# r stant is employed and wherein only the relatively low frequencycornponents of the input signal will be effective to vary the resistance of the thermistor.

Stillanother object is to provide an apparatus as above described whereini'tbe unknown signal is applied to an impedanceV brdge,}incorporating the thermistorV as one element thereof, through a low-pass'lilter and wherein aV l.relatively high frequency alternating current is ,used to polarize or excite the bridge circuit, a high-pass'lter being interposed between thebridgeand a bridgeV balance indicatingmeans so as to' preventf'the input 4signal from `cator. i 1 l v i Further objects will become apparent from the follow- Yin 'the lghtff the accompanying drawing. l

Intthedrawingg Y apparatusH embodyingmy invention.:4 A, j

sto' begmeasured', sl applied across the Ainput :ter-

directly affecting the reading ofthe bridge balance indi- Y onjunction with; thez ing description an'dclaims,r particularlywhen considered 65 .ift

2,981,887 Ptented Apr. 25, 1961 ICC thermally sensitive resistance 12. of thek type commonly known as a thermistor. The opposite end of the thermistor is connected to ground through a suitable voltage source, shown as a battery 13;

Thermistor 12 serves as one element of an A.C. im'- pedance bridge 14 which includes a pair of input termi:- nals 15 and 16 and a pair of bridge-balance-indicating or output terminals 18 and 19. As is shown in the drawing, therm'stor 12 is connected between the output terminal 19 and the input terminal 16 while a variableresistor 20 of substantially similar resistance is incorporated. in the corresponding opposite legbetween the terminals 18 and 16. The etectivenessof this apparatus for Vmeasuring the R.M.S. value of rlow frequency signals is to a large extent dependent upon the use of a thermistor 12 having a relatively long thermal time constant, preferably ofthe order of several minutes. While other thermistors probably would. be suitable, one such thermistor which is commercially available and has been found extremely satisfactory for use in this circuit is that known as the type WEl4B. r

The bridge is polarized or excited by a source of rela- `tively high frequency energy shown as a 3'5 kilocycle generator 2.1 connected between input terminal 15 and ground. As is well known when the impedances in the bridge are properly balanced, there will be no potential dfference appearing between the output lterminals 18 and 19 while, conversely, under unbalance conditions a 35 kilocycle voltage, the amplitude of which will cor'- respond tothe extent of unbalance, `will appear across these output terminals. Inv order to determine the balance condition of the bridge, the output terminals 18 and 19 are connected through a balanced type of high-pass filter 23 to the input grids'of `a pair of vacuum tubes 24 and 25, connected inrpush-pull in such fashion as to amplify any unb'alance voltage `appearing across the terminals 18 and 19. Inorder to indicate any unbalance, the output from one of these tubes may be applied to a single ended amplifier 26, which further amplies the un- "balance voltage, th'evalue of such amplified voltage being indicated bya suitable RF type volt meter, as is indicated at 27, for example." 1

While the tlow-'pass-filt'er 6 will-ordinarily beveffective to reinoveiany'l highV frequency components-fromf-the input signal, sudden, sharp pulses might occasionally be of sufficient amplitude and duration that theywould pass a I indicating vmeans v27. To'elminate thisY possibility, the

output ,from the cathodegfollower 84 is.`als`o'applied to "'r'esstor 2.0 through an 'isolating resistor 28 so that this output, isA in effect, applied in parallel across both resistor VwZtl and thermistorlz. Thus thefoutput of; theca'th'ode follower Ycannot "directly affect-the bridge balance indi cator,'but canonly indirectly affect it through its heating effect upon the thermistor 12. y k lnorder to evaluate the heating eflecrt,.that is; the "effec'tivey amplitude of the input signals;y 'upon "the thermistor 41,2, `apparatus is provided for comparingV thisV heating effect `with that producedgby alow Vfrequency alternatingl current `sourcelof. known `eective amplittlde;` [To thisl jen'dtthe movable yarm 220i lselector, switch-i4 .mayx be l Y 3 shifted into contact with the terminal 30 of the switch which is connected to such a suitable source of 60 cycle alternating current voltage. As shown, this voltage is obtained 'from the movable arm of a potentiometer 31 connectedto the output of a suitable transformer 32 which is in turn adapted to be connected to a conventional source of 60 cycle alternating current. The effective amplitude of the voltage at terminal 30 may be indicated by means of any conventional A.C. volt meter 34 calibrated to read the R.M.S. value of the 60 cycle sine wave voltage.

Cathode resistor may incorporate or be in the form of a potentiometer having a movable contact 35 by means of which the bias appearing on the grid 7 of the cathode follower may be adjusted to the desired normal or nosignal value.

Depending upon the physical layout of the components which make up the bridge, it may be found that a variable condenser 36 may be provided betweenone of the terminals 18 or 19 and'ground so as to balance out any difference in stray capacities between the two sides of -the bridge and ground.

As shown in the drawing, the lowand high-pass filters 6 and 23 are illustrated as being made up of a` series of individual resistance-capacity networks arranged in series. Obviously any known type of filter having the desired pass characteristics could be employed without af- Vfecting the operation of the unit. Obviously, too, a wide choiceofbridge balance amplifying and indicating arrangements is available to the designer of such equipment, the arrangement shown being merely illus- `trative of the general type which should be employed. The selection of components will, of course, depend upon the particular tubes used and will follow known engineerving design practices, well within the knowledge of those skilled in the electronic art.

In operation, after ak suitable warm-up period has elapsed, the movable contact arm 29 of switch 4 is brought into contact with the switch terminal 37 so as to ground the grid of the cathode follower with respect to any input signals. The normal or no-signal current `which will then iiow through the cathode follower 8 will then be adjusted, by shifting the movable arm 35 and thereby controlling the bias on the grid 7 of the cathode follower, so that the voltage ldrop across cathode resistor v10 will exactly balance the voltage of the battery 13. This willinsure that only the alternating components of the input signal will affect the balance of the bridge net- `work and will consequently keep thecur-rent flowing through the thermistor 12 toa minimum value. Switch arm 29 will then be shifted to the desired terminal of the step attenuator 3, 4for example, to the terminal 38, as in- ,dicated in the diagram, so that the input signal appearing across input terminals 1 and 2 will be passed bythe cathode follower to the terminals 18 and 19 of the bridge network and thence through the thermistor 1'2 andbalancing resistor 20 in parallel. resulting current flowing through thermistor 12 will vary its resistance and, as a result, will ,throw the bridge network out of balance. Under theseconditions there will be a 35 kilocycle voltage appearing ,across the output terminals 18 and 19, which voltage will beV applied through lthe high-pass filter 23 to the amplifiers 24-and 26 and will thereby produce a deflection of the balance indicating'meter 27. Resistor/20-will thereupon be adjusted The heatingeffect of the so as to restore the bridge tube balance, as indicated by the meter 27. AsV previously indicated, thermistor 12l has afrelatively long thermaltimegeonstant s ,o thaty itj ,will not respond to relatively short fluctuations in the outputofithe cathode follower but will gradually attain a temperature andv resulting resistance which will beproportional t'o the R.M.S. or effective amplitude of-the -inputsignal. Once'this stable condition has been reached, with .the bridge balanced, switch arm 29 is thrown1to terminal Yltljand 'potentiometer 31. isV adjusted until the '4 l bridge is again restored to balance. Since the effective or R.M.S. value of a voltage, regardless of its wave form, is a measure of the heating eiect of that particular current, the effective or R.M.S. value of the 60 cycle voltage appearing at terminal 30 when balance has been thus restored, is also the effective or R.M.S. value of the input signal voltage and may be directly read from meter 34.

Obviously in order to avoid nonlinear effects which might otherwise obtain, where the signal voltage to be measured has an unduly high value, the desired portion of this voltage may be selected by switching switch arm 29 to an appropriate one of the other contacts 39, 40 or 41 of the attenuator.

As previously indicated, theparticular components illustrated in the drawing are intended merely to be representative of many equivalent components well known in the art and are shown in this case for'illustrative purposes only. Obviously many changes and substitutions could be made without affecting the operation of the apparatus and without departing from the spirit and scope of this invention as defined by the appended claims.

I claim:

l. In an apparatus for determining the effective amplitude of a low frequency alternating voltage of unknown wave form, an impedance bridge having a pair of bridge input terminals and a pair of bridge-balance-detecting terminals, means for applying a high-frequency voltage to the input terminals and means connected to the balance detecting terminals for indicating any potential difference therebetween, said bridge including athermistor connected between one of said balance detecting terminals and one of said input terminals and an adjustable balancing resistor connected between the other of said balance detecting terminals and said one input terminal for balancing the impedance of said thermistor, and means connected to said bridge for applying a low frequency alternating voltage, the effective amplitude of which is to be determined, in parallel across said thermis- -tor and said balancing resistor.

2, In an apparatus for determining the effective amplitude of a low frequency alternating voltage of unknown `wave form, an impedance bridge having a pair of bridge input Vterminals and a pair of bridge-balance-detecting terminals, means for applying a high frequency voltage to the input terminals and means connected to the balance detecting terminals for indicating anyV potential difference therebetween, said bridge including a thermistor connected between one of said balance detecting terminals and one of said input terminals and an adjustable balancing resistor connected between the other of said balance detecting terminals land said one input terminal for balancing the impedance of said thermistor, a cathode follower including a control grid and a cathode, and having a load resistor in series with said cathode, means connecting said thermistor and said balancing resistor, parallel to one another, across said load resistor, and means for applying a low frequency signal, the effective amplitude of which is to be determined, to the *input terminalsv and means connected to thebalancede-g tecting terminals for indicating'any potential differencef, therebetween, said bridge including a thermistorconnected between oneof said balance'detecting terminalsk and one,` of said input terminals` and an adjustable balancing rcf sistor connected between the otherof said balance detecting terminals and saidoneinputgterminal for balancing l f V.the impedance of said thermistor, 'atcathode followergin-T -cluding a control gridand va cathode, and having a load resistor in series with.: said cathode, Y means connecting said thermistor and said balancing resistor, parallel to one another, across said load resistor, and including a direct current voltage source arranged to neutralize the normal D.C. voltage appearing across said load resistor under 11o-signal conditions, and means for applying a low frequency signal, the eiective amplitude of which is to be determined, to the grid of said cathode follower.

4. In an apparatus for determining the eifective amplitude of a low frequency alternating voltage of unknown wave form, an impedance bridge having a pair of bridge input terminals and a pair of bridge-balance-detecting terminals, means for applying a high-frequency voltage to the input terminals and means connected to the balance detecting terminals for indicating any potential difference therebetween, said bridge including a thermistor connected between one of said balance detecting terminals and one of said input terminals and an adjustable balancing resistor connected between the other of said balance detecting terminals and said one input terminal for balancing the impedance of said thermistor, a low-pass lter, a cathode follower connected to the output of said low-pass lilter and including a load resistor in series with the cathode thereof, means connecting said thermistor and sai-d balancing resistor, parallel to one another, across said load resistor and including a D.C. voltage source arranged to neutralize the normal D.C. voltage appearing across said load resistor under no-signal conditions, and means for applying a low frequencysignal, the effective amplitude of which is to be determined, to the input of said lowpass filter.

5. In an `apparatus for determining the eiective vamplitude of -a low frequency alternating voltage of unknown wave form, an impedance bridge having a pair of bridge input terminals and a pair of bridge-balance-detecting terminals, means for applying a high frequency voltage to the input terminals and means including a high-pass lter connected to the balance detecting terminals for indicating any potential diiference therebetween, saidv bridge including a thermistor connected between one of/said balance detecting terminals and one of said input terminals and an adjustable balancing resistor connected between the other of said balance detecting terminals and said one input terminal for balancing the impedance of said thermistor, and means connected to said bridge for applying a low frequency alternating voltage, the effective amplitude of which is to be determined, in parallel across said thermistor and said balancing resistor.

6. In an apparatus for determining the elfective amplitude of low frequency electrical signals of unknown wave form, signal input -terminals to which said signals are adapted to be applied, a source of low frequency alternating reference voltage of known wave form and adjustable in amplitude, a low-pass filter, and means for selectively connecting the input of said low-pass filter to said input terminals or to said source of reference voltage, a cathode follower connected to the output of said low-pass filter, and a thermistor connected to the output of said cathode follower whereby the impedance of said thermistor will be responsive to the output of said cathode follower, said thermistor having a time constant which is relatively large compared to the period of the lowest frequency alternating voltage component of the input signal to be measured, an impedance bridge including said thermistor as one leg thereof, a high frequency voltage source for exciting said bridge, means connected to said bridge and including a high-pass lter and an indicator responsive to said high frequency voltage for indicating any unbalance of the bridge, a variable impedance included in said bridge for balancing said bridge when said low-pass filter is connected to said input terminals, and means connected to said low frequency voltage source for controlling the amplitude of said reference voltage to rebalance said bridge when said low-pass filter is connected to said low frequency voltage source, and means responsive to said reference voltage for indicating the effective amplitude thereof.

References Cited in the file of this patent UNITED STATES PATENTS FOREIGN PATENTS Great Britain Sept. 10, 1952 

